The present invention relates to a heatable steering wheel with electric heating wires arranged on a steering wheel rim and a casing covering the heating wires.
Within attempts to improve comfort for motor vehicle users, inter alia heatable steering wheels have been offered, in order to be able to bring the most important grip region of the steering wheel, i.e. the steering wheel rim, to a haptically pleasant temperature as quickly as possible. As quickly as possible means, here, quicker than is possible with the conventional heating devices for the interior of motor vehicles. The heating wires can be connected with the electrical supply system of the motor vehicle via the ignition switch of the motor vehicle or in a preprogrammed manner, the heating output being monitored and regulated in a known manner using temperature sensors.
The steering wheel casing in such heatable steering wheels frequently consists of leather which is placed around the steering wheel rim over the heating wires and is sewn along an edge. One can also arrange leather strips, provided with heating wires, on the steering wheel rim and sew them to each other along an edge. The leather casing here has the purpose of concealing the heating wires visually, of protecting the hands of the motor vehicle user from a direct contact with the heating wires which are heated to 60 to 70xc2x0 C. and to distribute the heat, emitted from the heating wires, to greater surface regions. It is obvious that between the latter functions and the basic problem of providing a pleasantly tempered steering wheel as quickly as possible, a contradiction exists for which there can be no optimum solution but rather only a compromise matched to the respective case of application.
In top class vehicles, for which so-called wooden steering wheels are offered to an increasing extent, hitherto an acceptable compromise has been missing for the arrangement of heating wires. In steering wheels in which the metallic steering wheel skeleton has a casing of solid wood in the rim area, the arrangement of heating wires is not possible. xe2x80x9cWooden steering wheelsxe2x80x9d, however, are also understood to mean embodiments in which the steering wheel skeleton is encased with half shells of laminate wood in the rim area, onto which a layer of veneer wood is applied externally, For reasons of manufacturing technique, such half shells together with the externally applied veneer wood must have a wall thickness of at least 3 mm, which owing to the poor thermal conductivity of the wood is too much for the rapid heating of the steering wheel rim which is aimed for. With heating wires arranged beneath a wood layer at least 3 mm thick, the outer layer of the steering wheel rim can scarcely be brought more quickly to the desired temperature than is possible via the conventional heating arrangements for the interior of the vehicle. For these reasons, xe2x80x9cheatable wooden steering wheelsxe2x80x9d have hitherto not been developed and offered.
The invention overcomes the above-mentioned difficulties and proposes a xe2x80x9cheatable wooden steering wheelxe2x80x9d in which the surface of the steering wheel rim can be brought to a haptically pleasant temperature at a similar speed to steering wheel rims encased in leather.
According to the invention the steering wheel casing consists of a support layer having good thermal conductivity and of a layer of veneer wood lying externally and connected with the support layer.
Hitherto, one proceeded from the assumption that the arrangement of veneer wood in the area of the steering wheel rim requires a support layer of wood, because otherwise a sufficient adhesive strength could not be achieved between the support layer and the externally applied veneer wood. However, it has been found that veneer wood can also be reliably connected with support layers which consist of material having good thermal conducting properties and which with a desirable small thickness for the passage of heat have a sufficient mechanical strength. According to the invention, the support layers preferably consist of glass fiber-reinforced plastic, into which in addition materials having good thermal conducting properties such as molybdenum disulphide and carbon black are embedded.